**5. References**

74 Biogas

corresponded to a low consistency value (5\*10-10). At the static yield stress of 24 Pa (Fig. 5), the flow behaviour index showed shear thickening fluid behaviour (n=1.41) and a limit viscosity of 22 mPa\*s. This also corresponded to a low consistency value (5\*10-4). As soon as the fluid was measured again, n decreased (0.70) showing a pseduoplastic behaviour and K increased (0.11) indicating that the consistency of the reactor material was higher. The limit viscosity was 17 mPa\*s. These results showing a time dependency and structure recovery strengthen the arguments for a thixotropic fluid behaviour of reactor B. Once the stirring has ended and the fluid was at rest, the fluid structure starts to rebuild. Therefore, the viscosity become time dependent. This information is important to consider for biogas reactor

Herschel-Bulkley Ostwald Bingham

0 n R2 n R2 <sup>0</sup> R2

A 0.24 1.06 0.003 0.93 0.69 0.35 0.84 0.21 0.92 B 2.57 3.40 5\*10-10 0.45 0.08 2.28 0.002 1.88 0.12 C 2.89 0.59 0.42 0.99 0.44 1.23 0.99 6.36 0.95 D 0 0.65 0.04 0.88 0.64 0.04 0.87 0.33 0.95 E 2.38 0.49 0.98 0.96 0.39 1.98 0.96 8.31 0.91 Table 4. The results obtained from mathematical modelling of rheogram data of fluids from reactors A−E. 0: yield stress (Pa); n: flow behaviour index; : Consistency index; R2:

Also, fluids from reactor C and E were hard to define from modelling of the rheogram data because they gave indications for fluids being between pseudoplastic and Bingham plastic

The biogas reactor fluids investigated were behaving viscoplasticly, since they had yield stress and one of them was also thixotropic, due to its partial structure recovering. However, the reactor treating slaughterhouse waste was very close to act as a Newtonian fluid. Also, there was a difference in dynamic- and limit viscosities depending on the substrates used. The results demonstrated that similar TS values did not necessarily correspond to similar flow and viscosity behaviours. Nor, did biosludge from two different Swedish paper mill

To encounter problems related to involvement of new substrates and/or co-digestions in existing facilities, investigations for possible viscosity changes are needed. Ongoing research will hopefully provide an important basis for predictions of changes in rheology linked to the composition of the organic materials, which are translated in the process. This is important in order to achieve proper designs in relation to possible variation in substrate mixes in conjunction with new constructions, but also to better control material flows in the

behaviours, i.e. the 0-values were >0 (2.89 and 2.38) and n <1 (Table 4).

industries with similar TS show similar viscosity values.

existing facilities to avoid disturbances in the reactor performance.

performance, e.g. when applying semi-continuous mixing.

regression coefficient.

**4. Conclusion** 


**1. Introduction** 

production of wastewaters.

**4** 

*Spain* 

**Influence of Substrate Concentration** 

**on the Anaerobic Degradability of** 

**Two-Phase Olive Mill Solid Waste:** 

*Instituto de la Grasa (CSIC), Avda. Padre García Tejero, Sevilla,* 

The evolution of modern technology for olive oil extraction has affected the industrial sector depending directly on the by-products obtained. The traditional three-phase continuous centrifugation process for olive oil extraction was introduced in the 1970s, notably to increase the processing capacity and extraction yield and to reduce labour. This three-phase manufacturing process of olive oil usually yields an oily phase (20%), a solid residue (30%) and an aqueous phase (50%), the latter coming from the water content of the fruit, which is usually defined as vegetation water. Such water, combined with that used to wash and process the olives, make up the so-called "olive mill wastewater" (OMW) and also contains soft tissues from olive pulp and a very stable oil emulsion (Borja et al., 2006). This process generates a total volume of traditional OMW of around 1.25 litres per kg of olives processed. Consequently, the three-phase centrifugation process caused an increase in the average mill size, a decrease in the total number of mills, increased water consumption and increased

The OMW composition is not constant either qualitatively or quantitatively and it varies according to cultivation soil, harvesting time, the degree of ripening, olive variety, climatic conditions, the use of pesticides and fertilizers and the duration of aging. The three-phase OMW is characterized by the following special features and components: intensive violetdark brown to black in colour; specific strong olive oil smell; high degree of organic pollution (chemical oxygen demand –COD– values up to 220 g/L); pH between 3 and 6 (slightly acidic); high electrical conductivity; high content of poly-phenols (0.5-24 g/L) and

The annual OMW production of Mediterranean olive-growing countries is estimated to ranging from 7 million to over 30 million m3. This huge divergence of results can partly be explained by the fact that the production of olives varies from one year to another due to weather conditions and plagues that can affect the olive trees. The average total production amounts approximately to 10-12x106 m3 per year and occurs over a brief period of the year (November-March). Spain produced 20% of the OMW of the Mediterranean basin (2-3x106

high content of solid matter (Niaounakis and Halvadakis, 2004).

**A Kinetic Evaluation** 

Bárbara Rincón and Rafael Borja

Yang, F. ; Bick, A. & Shandalov, S. (2009). Yield stress and rheological characteristics of activated sludge in an airlift membrane bioreactor. *Jour Membrane Sci,* Vol.334, pp. 83-90.
